Particle beam apparatuses have already long been used to obtain insights with regard to the properties and behavior of objects under specific conditions. One of these particle beam apparatuses is an electron beam apparatus, in particular a scanning electron microscope (also called SEM hereinafter).
In the case of an SEM, an electron beam (also called primary electron beam hereinafter) is generated using a beam generator and focused by a beam guiding system, in particular an objective lens, onto an object to be examined (also called sample). Using a deflection device, the primary electron beam is guided in a raster-type fashion over a surface of the object to be examined. In this case, the electrons of the primary electron beam interact with the material of the object to be examined. As a consequence of the interaction, in particular interaction particles arise. In particular, electrons are emitted from the surface of the sample to be examined (so-called secondary electrons) and electrons of the primary electron beam are backscattered (so-called backscattered electrons). The secondary electrons and backscattered electrons are detected and used for image generation. An imaging of the surface of the object to be examined is thus obtained.
Furthermore, it is known from the prior art to use combination apparatuses for examining objects, in which both electrons and ions can be guided onto an object to be examined. By way of example, it is known to additionally equip an SEM with an ion beam column. Using an ion beam generator arranged in the ion beam column, ions are generated which are used for preparing an object (for example removing a surface of an object or applying material to the object) or alternatively for imaging. In this case, the SEM serves, in particular, for observing the preparation, but also for further examination of the prepared or unprepared object.
Besides the image generation already mentioned above, it is also possible to analyze interaction particles in greater detail with regard to their energy and/or their mass. A method is known from mass spectrometry, for example, in which method secondary ions are examined in greater detail. The method is known by the abbreviation SIMS (Secondary Ion Mass Spectrometry). In this method, the surface of an object to be examined is irradiated with a focused primary ion beam or with a laser beam. The interaction particles that arise in this case in the form of secondary ions emitted from the surface of the sample are detected and examined by mass spectrometry in an analysis unit. In this case, the secondary ions are selected and identified on the basis of their ion mass and their ion charge, such that conclusions can be drawn about the composition of the object.
The prior art discloses an analysis unit embodied, for example, as an ion trap mass spectrometer. In the case of the known ion trap mass spectrometer, a storage cell is embodied as a Paul trap. It has a ring electrode, a first end cap electrode and a second end cap electrode. The ring electrode is arranged rotationally symmetrically about a first axis. The first end cap electrode and the second end cap electrode are likewise arranged rotationally symmetrically about the first axis. The ring electrode, the first end cap electrode and the second end cap electrode encompass an interior of the storage cell. The ring electrode has an opening through which secondary ions can be coupled into the interior of the storage cell. The ions are dynamically stored in the ion trap mass spectrometer on the basis of an alternating field. An electric quadrupole field is generally used as the alternating field. In order to measure the mass to charge ratio, the ions are excited by an excitation signal to effect oscillations, the frequency of which is dependent on the ion mass. The oscillation information is tapped off at the first end cap electrode and the second end cap electrode and evaluated. For this purpose, measurement currents that arise as a result of induced image charges are measured at the end cap electrodes.
Disturbing effects in the form of crosstalk currents occur, however, in the case of the ion trap mass spectrometer described above. In this regard, a first crosstalk current arises on account of a first interaction of the alternating field with the first end cap electrode. Furthermore, a second crosstalk current arises on account of a second interaction of the alternating field with the second end cap electrode. The first crosstalk current has a first frequency, a first amplitude and a first phase. Furthermore, the second crosstalk current has a second frequency, a second amplitude and a second phase. The crosstalk currents are usually greater than the actual measurement current by a multiple. At the first end cap electrode and at the second end cap electrode, a current (also called measurement signal hereinafter) composed of the actual measurement current and the crosstalk current is tapped off, and is generally amplified by a measurement amplifier. The measurement amplifier is often overdriven on account of the high crosstalk current, and so the amplified signal of the measurement amplifier yields no useable information about the stored ion.
A solution to this problem is known from the prior art. In this solution, the measurement signal is measured at the first end cap electrode. Furthermore, a first compensation current is provided, the first compensation current likewise having the first frequency and the first amplitude. Furthermore, the first compensation current has a first compensation phase, which is offset by 180° with respect to the first phase of the first crosstalk current. Afterward, the first compensation current is superposed with the first measurement signal in such a way that a first resultant signal arises. By filtering the first resultant signal, a first filter signal is ascertained, wherein the latter substantially comprises the first measurement current, that is to say the actual measurement signal. Afterward, the first filtered signal is amplified using a measurement amplifier and the amplified filtered signal is subsequently evaluated in order to determine the ion or the ions. The same is also analogously effected with the second measurement signal at the second end cap electrode.
The above-mentioned procedure is generally made available by a complex implementation using software. Direct digital syntheses (DDS) are used for this purpose. These are methods for generating periodic, band-limited signals in digital signal processing. However, the software is complex in its programming and often also has errors.
Therefore, it would be desirable to specify a device for mass selective determination of ions in which a compensation of the crosstalk current with a compensation current and downstream filtering of the resultant signal can be provided more simply.